Server :: Finding The Source Of The Email In The Main Queue?

I have a server (RHEL 4.7) with an mqueue that keeps growing over a two or three day interval. Then I have to manually flush the queue by removing all the mail in the spool directory as the sendmail flush command doesn't work. I've looked at the mail queue manually with a sendmail -bp command and it seems to show an occaisional mail item in the queue. On the other hand, there seems to be a lot more non-legitimate emails in the queue spool. how do I find out the source of the email in the main queue? Next, how do I stop it from building the queue? Next, is there anyway to automatically flush the queue once it builds if I need to?